In a typical broadcast environment, a radio signal leaving the transmitter may arrive at the receiving antenna via multiple paths due to reflections from surfaces that do not absorb the frequencies being transmitted, such as buildings, hills, etc. Since the multiple signals at the receiving antenna travel different distances, they can be out of phase with respect to each other, and thus may combine both subtractively and additively. The result is fluctuating signal strength, distortion and noise. This signal corruption is typically called multipath interference.
One way to avoid signal corruption due to multipath interference is to employ spaced apart antennas in a diversity system. The spaced antenna diversity system employs two or more antennas spaced at least one-quarter wavelength apart. When a selected antenna enters a multipath null, a location at which signals cancel each other as they combine at the antenna, a diversity circuit detects the null and switches the receiver to another antenna. On the premise that, in most cases, the other antenna will not be positioned in the multipath null, the noise including interference and distortion that would otherwise be heard by the listener may be reduced.
Typically, a diversity circuit will respond to a drop in the signal strength as represented by a DC voltage signal developed by the FM detector. A decision section of the circuit compares an instantaneous signal strength voltage signal to an attenuated average signal strength voltage signal. The circuit may also include a weak signal comparator to prevent switching to a less sensitive antenna when a weak signal is being received to avoid complete loss of the signal.
A control section of a circuit returns the receiver to the original antenna when the newly selected antenna is of equal or less strength than the original antenna. Such a section often includes control to disable the diversity circuit when the radio is in the tune, seek or scan mode of operation. An antenna switching section of the circuit includes transistor switches which bias PIN diodes that alternatively connect the antennas to the FM front end. The control section can be conveniently implemented by integrated circuits (ICs) as is the decision section of the diversity antenna circuit.
The previously known diversity circuits often employed a switching strategy that involved comparison of the received signal strength or quality to a predetermined threshold. However, thresholds which cannot be changed according to the environmental needs may induce excessive switching or insufficient switching depending upon the broadcast environment in the area. In addition, other FM diversity strategies employ two tuners in the structure which substantially increases the equipment, size and expense of operating the diversity antenna system.
When multiple signals of the same source combine subtractively at the receiving antenna, a sudden drop occurs in the signal strength level. If a receiving antenna is moving, as is common with radios in motor vehicles, a drop in signal strength is usually of short duration since it lasts only as long as it takes the antenna to pass through the null. However, antenna switches responsive to the drop in signal level may contribute to excessive switching and unnecessary switching to an antenna providing a signal of less quality before switching back to the better performing antenna for that broadcast environment. Such interruptions adversely affecting the sound quality perceived by the listener, and may be more annoying than the loss of signal strength.